Improving Genetics With Reproductive Biotechnology

Today, like never before, reproductive biotechnology allows us to make rapid changes in the genetics of food and fiber producing animals. Take, for example, the role of artificial insemination - one of the first reproductive biotechnologies to achieve widespread use - in enhancing milk production from the national dairy herd (Table 1).When used on a large scale, artificial insemination (AI) allowed

Gordon D. Niswender | Mar 01, 1999

Today, like never before, reproductive biotechnology allows us to make rapid changes in the genetics of food and fiber producing animals. Take, for example, the role of artificial insemination - one of the first reproductive biotechnologies to achieve widespread use - in enhancing milk production from the national dairy herd (Table 1).

When used on a large scale, artificial insemination (AI) allowed reliable progeny testing of dairy bulls. Producers used the resulting information to identify those bulls whose daughters were proven to be excellent milk producers. Semen from identified bulls was used to artificially inseminate large numbers of cows. As a result, in the 40 years following introduction of artificial insemination with frozen semen, milk production increased 265% per cow.

Rapid increase in milk production was possible because breeders focused primarily on a single trait. Controlled environment, improved nutrition and management of herd health also played roles in increased milk production in the dairy industry.

Unfortunately, improving genetics for production traits in beef animals proves more difficult. For example, should we select for a single trait like maternal ability, growth rate, lean meat, marbling, feed efficiency, carcass yield or tenderness? Or should we select for multiple traits or combinations of traits?

Beef producers do not agree on what traits are most important. Environmental conditions vary dramatically, which means that the "perfect" beef cow in the South is likely to be quite different from an ideal cow in the West.

However, it is also clear that once we identify specific, desirable genetic traits in beef animals, we can rapidly expand the presence of the desired genes in the general population.

For example, it's possible to produce 100,000 calves per year from a single bull via AI.

If superior genetic traits are expressed in females, several dozen calves can be produced annually by superovulation followed by transfer of the embryos to surrogate mothers. Splitting the embryos to produce identical twins can increase this number.

Even more calves can be produced by using ultrasound-guided removal of eggs, or oocytes, from cows, which can then be fertilized in a laboratory and transferred to surrogate mothers.

Even with all this potential, however, none of the above methods truly allow superior female genetics to have a major impact on very large numbers of animals.

We know each cow ovary contains approximately 130,000 oocytes. To take full advantage of this large pool of female genetic material, many researchers are trying to find methods to collect these oocytes and mature and fertilize them in the lab.

In the future, tens of thousands of calves might be produced from a single superior female this way. Some success has already occurred with mice, and success with species of economic importance should occur within the next decade.

Cloning The recent cloning of adult sheep stimulated a lot of interest in clones - individuals with identical genetic compositions. Cloned cattle and horses were first produced over a decade ago by splitting embryos eight to 10 days following fertilization (when the embryos had approximately 64 cells).

Using microsurgical procedures to produce multiple, genetically identical offspring, researchers soon discovered that each embryonic cell, or that cell's nucleus, was capable of developing into a new individual up to approximately the 128-cell stage of embryonic development. This led quickly to the use of nuclei from embryonic cells to produce dozens of cloned calves from a single developing embryo.

Advancement has been rapid, but the procedures are technically demanding and efficiency is poor. To date, cloning has not proven to be an economically viable method of producing large numbers of calves.

More recently, there has been a major breakthrough in animal cloning methods. In the case of the famous sheep "Dolly," cells were obtained from the mammary gland of a ewe and allowed to divide and grow under laboratory conditions.

As a second step, a mature oocyte was obtained from a ewe and the chromosomes/genetic material removed by microsurgery. The nucleus (which contained the genetic material) was removed from a mammary gland cell, then placed into the oocyte, replacing the removed genetic material.

After repeated tries, an embryo containing genetic material from the mammary cell began to develop. That embryo was placed into a surrogate mother. The procedure was performed 273 times before successfully producing the lamb named "Dolly" at a cost of hundreds of thousands of dollars.

Some researchers in this area are trying to produce large numbers of cloned calves that are genetically identical for use in embryo transfer. Additionally, since they are genetically identical, all of the calves originating from a single clone would be of the same sex.

Dolly's production was significant for two reasons. First, she was the first mammal produced with genetic material obtained from an adult cell. Second, this procedure's success made possible the development of a new, more efficient method for producing transgenic animals - animals that contain a synthetic, human-added gene.

After Dolly, the next step for these researchers was introduction of a human clotting factor gene into cultured mammary gland cells. Clotting factor deficiency causes the human disease called hemophilia.

Once researchers were sure the mammary cells contained the gene for the clotting factor, they removed the nucleus from several of these transgenic cells and placed each into a denucleated oocyte. Thus, the cloned ewes "Molly" and "Polly" were born.

The researchers hope these ewes will secrete large amounts of the clotting factor into their milk so it can be purified and used to treat hemophilia in humans. The term "pharming" applies to this type of research - when domestic animals are made transgenic to produce molecules useful for pharmaceutical purposes.

Researchers are now making numerous improvements in methods used for cloning, and it will likely soon be possible to produce hundreds to thousands of genetically identical individuals with this technology. However, the technology may never become economically viable for large-scale production of calves.

Sexed Semen Sexed semen is another promising reproductive biotechnology. Sexing semen involves the use of high-speed cell sorters to sort semen into batches containing approximately 90% X-chromosome (female producing) or Y-chromosome (male producing) bearing sperm. Many cows have been artificially inseminated with sexed semen, and the offspring are the predicted sex approximately 90% of the time.

This technology has a number of applications for altering cattle genetics. An obvious example is in dairy cattle. A dairy producer could breed the top milk producers with X-bearing semen to produce heifer calves, and the lower-producing cows with Y-bearing semen from beef bulls to produce bull calves. This way, producers could always get replacement heifers from the highest-producing cows and obtain a more valuable, beef-type crossbred bull calf from cows that are low producers.

In the beef industry, top-performing heifers could be bred with X-bearing semen to produce heifer calves as replacements, increasing the speed of genetic improvement within the herd. Another benefit would be a 5- to 7-lb. reduction in birth weights due to heifer calves being produced by first-calf heifers.

Another application could be in developing maternal and terminal lines. The use of sexed semen would allow development of a cowherd with excellent maternal traits (high reproductive efficiency, good milk production and good maternal behavior) to be bred to terminal-cross sires resulting in calves with good feedlot and carcass characteristics. With this scheme, it should be possible to do a much better job of matching cows to environment and still produce a more uniform calf crop that produces high-quality meat.

Advances in reproductive biotechnology are occurring rapidly. These procedures have great potential for altering the genetics of beef animals. The challenge comes in identifying those traits that are important enough to merit the application of this technology. As these procedures become more economically practical, they will play a very important role in improving the genetics and performance of beef cattle.